Ag alloys in which oxides such as Cd, Sn, and In are dispersed have been widely used as electrical contact materials for breakers in a conventional art. In particular, Ag alloys in which the Cd oxides are dispersed are suitable for such a kind of electrical contacts and are widely used for the breakers. However, the drawback of Cd compounds is that they are poisonous. Recently, therefore, there has been strong demand for the development of a Cd-free Sn- or In-dispersed Ag alloy as an electrical contact material which can replace Ag alloys containing Cd oxides. Thus, various materials have been developed for use in electrical apparatuses.
An electrical contact made of Cd-free Ag alloy is suitable for a relatively light load electrical apparatus in which temperature characteristics are important, or a light load apparatus such as a contactor in which contact resistance matters. However, such a Cd-free Ag alloy is inferior in terms of performance as compared with an Ag alloy containing Cd when it is used as a material of an electrical contact for a breaker in which a rated current equal to or more than 10 A is required. For example, at present, most of breakers in which a rated current equal to or more than 10 A and cut-off current equal to or more than 1.5 KA are required, and to which the present invention is mainly directed, use an electrical contact that contains Cd equal to or more than 10% by weight. Cd equal to or more than 10% by weight is contained in electrical contacts used in magnetic switches for a rated current equal to or more than 100 A or those for a forklift using a heavy load contact of a direct current voltage of 86 V and a rush current of 1.9 KA to 2 KA. On the other hand, an electrical contact made of Cd-free Ag alloy is commonly used for a low rated magnetic switch, relay, etc.
The characteristics which are required for an electrical contact for breakers are (1) a welding resistance characteristic, (2) an initial temperature characteristic, (3) a temperature characteristic after an overload test, (4) a temperature characteristic after an endurance test, (5) an insulation characteristic after a cutoff test, and (6) a wear resistance characteristic. When these characteristics are examined with a single material having the same chemical composition and microstructure, it is found that there is a trade-off between some characteristics: between (1) and (2), for example. Therefore, when an electrical contact made of a single material is used, it is necessary to sacrifice one characteristic that is in the trade-off relation with the other. The first characteristic that must be improved so that the electrical contact made of the Cd free Ag alloys replaces the electrical contact containing Cd for the breakers is the welding resistance characteristic. The second characteristic is the temperature characteristic that is in the traded-off relation with the welding resistance characteristic in the same material. It is important that the breakers can be stably used in the areas of relatively high rated current and breaking capacity. Also, it is necessary to improve the wear resistance characteristic and the breaking characteristic to a certain level. Therefore, trials have been made to fabricate a composite contact by combining one material having an excellent characteristic with another material having another excellent characteristic that involves a trade-off between them. Among the trials, a conventional art relatively close to the present invention will now be described.
For example, examples of making such composite materials are described in Japanese Unexamined Patent Application Publication Nos. 58-189913 and 62-97213. In the electrical contacts described in the publications, a material having excellent wear resistance and welding resistance characteristics is used as the surface layers, and a material having an excellent breaking characteristic is used as an inner layer. The contrivance in both of the inventions is such that in electrical contacts, an Ag—Sn—In based alloy is arranged as a surface layer and pure Ag or a high conductive material containing a large amount of Ag is arranged as an inner layer so that arc cutoff is improved.
The former is designed such that in consideration of the arc cutoff during a short circuit, the surface layer is considerably thicker than the inner layer (the inner layer has a thickness of about 300 to 1200 μm while the surface layer has a thickness of about 100 to 300 μm.), and in consideration of the case in which the surface layer is consumed, a concavo-convex joint is formed on the boundary between the surface layer and the internal layer so that a part of the surface layer is left so as to be continuously used after the surface layer above the joint has been consumed and damaged by the arc. On the other hand, in the latter, the surface layer is thinner than that of the former (10 to 200 μm). However, the amount of the oxide dispersed in the surface layer is increased in consideration of the arc cutoff during the short circuit so that the hardness of the surface layer is improved (For example, when the surface layer is made of an Ag alloy in which the oxides of Sn and In are dispersed, the total amount of the oxides are equal to or more than 10% by weight). Since Ag or an alloy containing a large amount of Ag is used for the internal layers in the electrical contacts, it is considered that the arc cutoff time during the short circuit is surely short. However, it is apprehended that, when the electrical contacts are used for a contact for a breaker that cuts off a large current equal to or more than 6 KA, a large arc is generated such that a welding accident may be generated right after the surface layer has been consumed and damaged. Also, a work of forming a concavo-convex portion on the matching surfaces of the upper and lower Ag alloy materials and matching the upper and lower Ag alloy materials is disadvantageous in terms of productivity and economy.
Also, in Japanese Unexamined Patent Application Publication No. 61-114417, a composite electrical contact made of an Ag alloy containing Sn and In, in which the amount of the oxides of Sn and In in the surface layer, in particular, the amount of the Sn oxide is smaller than that in an internal layer, is disclosed Since the surface layer of the contact thus made is less hard than the internal layer, when it is used as a contact for a breaker, the wear resistance characteristic of the surface layer is inferior such that the welding accident may easily occur. Japanese Unexamined Patent Application Publication No. 10-188710 discloses another type of two-layered composite electrical contact. The electrical contact according to the invention is for a breaker suitable for a rated current equal to or less than 100 A. The two layers are an outer circumference layer generally having an excellent welding resistance characteristic and a central layer having an excellent temperature characteristic. Both of the layers are mainly made of an Ag alloy in which the oxides of Cd, Sn, and Ni are dispersed. In the contact, the welding resistance characteristic and the temperature characteristic are adjusted to appropriate levels by controlling the hardness of the two layers and the ratio of the two layers with respect to the contact surface areas thereof. The hardness of the outer circumference layer of the electrical contact is equal to or more than 135 based on micro Vickers standard. The hardness of the central layer is less than 135. An electrical contact according to the invention is suitable for a breaker of a rated current equal to or less than 100 A. However, this contact has the drawback of poisonousness because it contains a large amount of Cd.
It is an object of the present invention to provide an electrical contact which is made of a Cd-free unpoisonous Ag alloy and in which a welding resistance characteristic and a temperature characteristic that involve a traded-off between them are appropriately controlled, and in particular, an electrical contact useful for a breaker having a rated current equal to or more than 10 A and cut-off current equal to or more than 1.5 KA, and to provide a breaker using such contact.